South Korea’s navy fired warning shots to drive away North Korean patrol boats from the disputed inter-Korean sea border, amid tension over the sinking of a Seoul warship, officials said Sunday.

The warning shots were fired late Saturday when two North Korean patrol boats violated the Northern Limit Line (NLL) border and strayed into South Korean waters, the South’s Joint Chiefs of Staff office said.

A spokesman for the Joint Chiefs of Staff told AFP the North’s patrol boats retreated without responding to the fire. No casualties occurred.

It is the first time that shots have been fired at the tense sea border since the mystery sinking of a South Korean warship on March 26.